Collision Induced Dissociation of Metal Cluster Ions: Bare Aluminum Clusters, Al sub n sup + (n=3-26).

The collision induced dissociation of aluminum clusters Al sub n sup + (n=3-26) by argon, at a center of mass collision energy of 5.25eV, has been studied using a low energy ion beam apparatus. Product branching ratios and collision induced dissociation cross sections are presented and discussed. The main product is Al sup + for the smaller clusters and Al sub (n-1) sup + for the larger ones. The cross sections rise to a peak at Al sub 6 sup + - Al sub 9 sup + and then decrease with increasing cluster size. Cross sections for Al sub 7 sup +, Al sub 13 sup +, Al sub 14 sup + and Al sub 23 sup + are significantly smaller than their neighbors. A crude kinetic model is used to derive approximate cluster ionization potentials from the product branching ratios. The IPs initially rise with cluster size, peak at Al sub 6 and then decrease. The IP of Al sub 7 is particularly low and there is a sharp drop in IP at Al sub (14) where the IP falls below that of the atom.